An optical transmission signal fades with distance when traveling through any type of optical fiber telecommunication system and, thus, needs amplification. In this regard, optical fiber amplifiers are used to transform a weak input optical transmission signal into a strong output optical transmission signal. Optical fiber amplifiers contain optical fibers with cores doped with certain rare earth elements, such as, erbium, that amplify light at certain wavelengths. The amplified wavelengths depend primarily on the rare earth dopant and on the fiber composition. Typically, a rare earth doped optical fiber amplifier utilizes a light source from an external laser, such as a semiconductor pump laser, to excite the dopant atoms in the optical fiber from a ground state to a higher energy level, whereby light from an optical transmission signal having a signal wavelength can stimulate these excited atoms to emit their excess energy as light at the signal wavelength, thus resulting in an amplified optical transmission signal. The degree of amplification depends on the excitation power input, as well as on the excitation wavelength. Standard erbium-doped fiber amplifiers amplify light having a wavelength in the range of about 1520 and 1610 nanometers and are usually pumped by commercially available semiconductor pump lasers that emit light at either 980 or 1480 nanometers.
In telecommunication systems, such as metropolitan area networks (MANs), which can span a geographical area the size of a city, a plurality of optical fiber amplifiers are optically linked together. Each optical amplifier in such a network must satisfy a number of rigorous technical requirements in order to assure reliable and accurate communications within a fiber-optic network. Further, each optical amplifier in a network has a unique set of components that require individual fabrication and testing. Ordinarily, in many long-distance applications, each optical amplifier in the network is spliced to a transmission line fiber and further the optical components that comprise each of the amplifiers are also spliced together. Since splicing results in a permanent connection and generally results in a lower attenuation of an optical signal-splices are the preferred way to join lengths of fiber in long-haul telecommunication systems. To provide good optical performance, the optical fibers used in such amplifiers and the various optical components contained thercin need to be optically connected such that there is minimal signal loss. Because the manufacture of optical amplifiers typically requires optically connecting various sections of fiber, many points of failure are possible. Accordingly, any failure in a network generally requires shutting down the network to access one or more of the failed components. Troubleshooting the cause of a network failure is time consuming, hence, the cost of a network failure can nm into hundreds of thousands, if not millions, of dollars in lost communications.
Typically, during manufacturing, optical fiber amplifiers are individually assembled with a particular focus on an amplifier's location and function within a network. For instance, an optical amplifier can be used as an in-line amplifier between cable segments, as a post-amplifier or an output amplifier to increase transmitter output, as a pre-amplifier or an input amplifier to increase receiver sensitivity or combined with other components to offset high losses. Such individualized amplifier manufacturing techniques have been one of the contributing factors in the high cost of optical fiber amplifiers. When a newly assembled optical fiber amplifier fails to meet the desired technical specifications and needs reworking, such failures lead to significant delays in the production process. In addition, when manufacturing more than one type of amplifier for a network system, such delays in reworking are multiplied and can result in significant monetary loss due to the precise assembly and testing procedures utilized.
In light of the foregoing, it is desirable to simplify the manufacturing and testing process for optical fiber amplifiers and, also, increase amplifier production with minimal rework. Further, it is desirable to provide a method for coupling multiple pump lasers to a doped optical fiber amplifier in order to provide high output power. Also, it is desirable to simplify the assembly of different types of amplifiers, where common components in the different amplifiers can be easily utilized in assembling more than one amplifier. Furthermore, it is desirable to reduce transmission equipment costs, improve line reliability and simplify maintenance and operation functions of optical communication systems.